Duoxu is a terminally endangered and virtually undescribed Tibeto-Burman language, spoken in the historically multi-ethnic and multi-lingual Miǎnníng county of Sìchuān province in the People’s Republic of China. Until recently, Duoxu was known only through a 740-word vocabulary list in the Sino-Tibetan vocabularies Xīfān Yìyǔ [Tibetan-Chinese bilingual glossary], recorded in Chinese and Tibetan transcriptions in the 18th century, and a grammatical sketch (Huáng & Yǐn 2012). Researchers who have worked on the language (Nishida 1973, Sūn 1982, Huáng & Yǐn 2012) have expressed different views about the features and the genetic position of Duoxu, variously viewing it as (1) closely related to Lolo-Burmese languages (Nishida 1973), (2) closely related to Ersu and Lizu, two neighboring languages that are currently classified as members of the Qiangic subgroup of the Tibeto-Burman language family (Sūn 1982), or (3) distantly related to those two languages and to Qiangic languages at large (Huáng & Yǐn 2012).The Duoxu language is critically endangered and urgently requires documentation. It is of great value for our understanding of the linguistic diversity of the region, and of its linguistic history. It is also of great value as a modern reflection of a language that was recorded in the 18th century. This paper makes a significant contribution in all these areas. Based on new fieldwork with all remaining elderly Duoxu speakers, this study provides newly collected data and a new analysis. It compares the newly collected data with the 18th-century attestations of Duoxu as well as with its two putative sister languages Ersu and Lizu. The conclusion of the study is that Duoxu is closely related to Ersu and Lizu, with superficial differences attributed to long-standing and on-going contact influence from Southwestern Mandarin.